1. Technical Field
The present invention relates to differential amplifiers in general, and in particular to complementary metal-oxide semiconductor (CMOS) differential amplifiers. Still more particularly, the present invention relates to an apparatus for biasing a CMOS differential amplifier.
2. Description of Related Art
Differential amplifiers are commonly employed in various electronics applications for providing an output in response to a pair of differential inputs. Differential amplifiers can also be readily adapted to function as an operational amplifier, a sense amplifier, a comparator, etc. Basically, differential amplifiers are utilized where linear amplifications with minimum distortions are desired.
However, differential amplifiers typically can only operate over a relatively narrow range of common-mode input voltages. As a differential amplifier is forced to extend beyond the range of common-mode input voltages, the differential-mode gain drops off sharply.
There are prior art techniques for improving the common-mode input voltage range of differential amplifiers, but such techniques use biasing schemes that are uncompensated for variations in common-mode voltage, supply voltage, temperature, and process. Because of the lack of compensation, the practical common-mode range of the differential inputs is somewhat less than the full rail-to-rail range. In addition, some of the prior art techniques rely on the usage of saturated current sources for biasing differential amplifiers, which can result in a reduction of the bandwidth of the differential amplifiers.
Consequently, it would be desirable to provide an improved apparatus for biasing a differential amplifier such that its common-mode input voltage range can be extended to as far as the rail-to-rail voltage, while maintaining the differential-mode gain of the differential amplifier at a high level.